1. Field of the Invention
The present invention relates to a self-start synchronous motor, a method for manufacturing the same and a compressor.
2. Description of Related Art
For example, JP-A-2001-73948 (Patent Document 1) discloses a conventional self-start synchronous motor, which comprises a stator having a stator core and windings wound on the stator core, and a rotor having an outer diameter slightly smaller than that of the inner diameter of the stator and arranged inside of the stator. The rotor is provided with a rotor core, multiple conductor bars circumferentially arranged and embedded in the outer peripheral portion of the rotor core, end rings (short-circuit rings) provided on opposite ends of the conductor bars and forming a squirrel cage conductor in cooperation with the conductor bars, and a plurality of permanent magnets arranged on the inner peripheral side of the conductor bars and embedded in the rotor core. Further, the conductor bars and the end rings are integrally incorporated with each other by aluminum-die-casting.
Meanwhile, for example, JP-A-2002-291211 (Patent Document 2) and JP-A-2003-289655 (Patent Document 3) disclose conventional methods of manufacturing rotors for induction motors.
In the manufacturing method disclosed in the patent document 2, aluminum conductor bars and aluminum end rings are joined together through frictional agitation in order to form a squirrel cage conductor within a rotor core which is composed of silicon steel plates stacked one upon another. This manufacturing method causes no blow holes in the conductor bars and the end rings in comparison with a method in which the conductor bars and the end rings are formed by aluminum die-casting, and accordingly, the rotor can be well-balanced and a satisfied electric performance can be exhibited therefrom.
In the manufacturing method disclosed in the patent document 3, copper conductor bars and copper end rings are joined together through frictional agitation so as to form a squirrel cage conductor within a rotor core. This manufacturing method uses the conductor bars and the end rings, which are made of copper having a low specific resistance, and accordingly, the secondary current resistance can be reduced, and as a result, the rotor loss can be reduced.
However, in the self-start synchronous disclosed in the Patent Document 1, it has been found that the permanent magnets could deteriorate since the conductor bars and the end rings are integrally incorporated with each other by aluminum die-casting and the permanent magnets are subjected to heat at a temperature higher than the melting point (660 deg. C.) of the aluminum material.
Further, in the self-start synchronous motor disclosed in the patent document 1, since the conductor bars and the end rings are made of an aluminum material, the resistance value of the secondary conductor depends upon a specific resistance of the aluminum material and a cross-sectional area and a length of the conductor bars, and accordingly the positions of and the size of the permanent magnets embedded in the secondary conductor at the inner peripheral side are univocally determined, thereby it is difficult to further enhance the efficiency.
In a design, the outer surface areas of the permanent magnets can be increased by arranging the permanent magnets on the outer peripheral side of the rotor, thereby it is possible to enhance the efficiency. Further, should the secondary conductor be formed from a copper material having a specific resistance lower than that of the aluminum material, by copper die-casting, deterioration of permanent magnets would be further accelerated since the melting point of the copper material is extremely high, that is, 1,083 deg. C.
Meanwhile, the patent documents 2 and 3 does not concern a self-start synchronous motor but an induction motors, and fail to disclose or suggest the above-mentioned problems caused in the case of the application of the methods to the self-start synchronous motor.